The present invention relates to pressure sensitive adhesive tapes and, more particularly, to silicone compositions suitable for forming primer layers for pressure sensitive adhesives. The present invention further relates to a primer and adhesive system for a pressure sensitive adhesive tape wherein the primer and adhesive compositions each comprise a similar phenyl-modified silicone component.
Silicone pressure-sensitive adhesives (PSA""s) are primarily composed of a mixture of high molecular weight, silanol-functional organosiloxane polymers and silanol-functional polymer resins. These polymer/resin blends are often further batch-compounded in solvent solution by the catalyzed condensation reaction (in situ) of the silanol-functional reactants. Methyl-based silicone PSA""s incorporate a polymer component based on repeated dimethyl siloxane building blocks. Phenyl-based silicone PSA""s incorporate a polymer component based on a combination of both dimethyl siloxane and diphenyl siloxane building blocks. The presence of pendent phenyl functionality in the silicone polymer backbone increases both the higher temperature performance and the electrical resistance properties of the adhesive.
Silicone PSA""s are used for coating high-temperature film substrate, also called base films, such as polyester, polyetherimide, polyimide, PTFE (polytetrafluoroethylene), and FEP (fluoroethylene propylene) in the preparation of high-performance industrial tapes. These tapes may be used for electrical insulation, electronic assembly masking, flame and plasma spray masking, EMI/RFI shielding, and splicing operations.
These processed tapes are self-wound, resulting in a number of dispensing issues for the tape manufacturer. Handling difficulties are created as a result of insufficient xe2x80x9canchoragexe2x80x9d of the adhesive layer to the substrate film. When the tape is unwrapped during slitting operations or in automated dispensing operations, the adhesive may delaminate from the base film and transfer to the back-side of the wrapped tape, splitting the adhesive layer. This phenomenon is known as xe2x80x9cleggingxe2x80x9d. xe2x80x9cWebbingxe2x80x9d phenomena may also occur during slitting operations when the adhesive layer resists clean slitting, resulting in an adhesive xe2x80x9cwebxe2x80x9d that promotes delamination off the base film along the slit-edges.
Both xe2x80x9cleggingxe2x80x9d and xe2x80x9cwebbingxe2x80x9d can be eliminated if the adhesive is sufficiently primer-bonded to the surface of the film substrate. A primer which has been previously applied to the surface of a film substrate acts as a tie-layer for securely anchoring the adhesive layer to the substrate.
Typically, the surface energy of a silicone PSA is extremely low, usually within the range of 20-26 mNm-1. This surface energy is in the same general range as traditional silicone release coatings which use this unique property for providing an easy-release surface for label stock. Organic-based (non-silicone) PSA""s are high surface energy systems and consequently will not xe2x80x9cwet outxe2x80x9d onto lower energy surfaces, thus accounting for the ease-of-release characteristic. On the other hand, silicone PSA""s do not readily release from a silicone release coating because their low surface energy allows them to xe2x80x9cwet outxe2x80x9d onto the surface of the release coat. It has been found that some silicone release coatings, such as those having a silanol gum with a dimethyl-siloxane polymer component, make excellent primers for most silicone PSA""s in the preparation of high-performance industrial tape. Such silicone release coatings are widely used in industrial tapes for priming silicone PSA filmstock.
Although the technique of priming with primers comprising a dimethylsiloxane silanol gum has been a standard in the industry for many years, it has been found in practice to be only feasible for methyl-based silicone PSA""s. However, these primers and other similar commercially available primers do not provide the anchorage necessary for phenyl-based silicone PSA""s. A need therefore exists for an effective primer for phenyl-based PSA""s.
The present invention relates to silicone primer compositions that provide excellent anchorage for phenyl-based silicone pressure sensitive adhesives used in the preparation of PSA tapes. The primer compositions comprise a silanol chainstopped phenyl-modified siloxane gum component, preferably a dimethyl-diphenyl siloxane gum component. The relative amount of phenyl substituents in the phenyl-modified silicone primer may vary depending upon the relative amount of phenyl substituents present in a pressure sensitive adhesive composition which is to be used in conjunction with the primer. More preferably, the amount of phenyl substituents in the primer matches the amount of phenyl substituents in the adhesive. Most preferably, the amount of phenyl substituents in the primer is within about ten percent of the amount of phenyl substituents in the adhesive.
According to one embodiment, the amount of phenyl substituents in each of the primer and adhesive is preferably between about 5.0 and about 5.5 mole percent. One particular primer and adhesive system in accordance with an embodiment of the present invention comprises a dimethyl, 5.3 mole percent diphenyl, silanol chainstopped silicone gum in both the primer and the adhesive.
Very thin layers of the primer are used between the adhesive composition and a supporting substrate. The primer compositions of the present invention, when used according to the present invention, virtually eliminate all tendencies of phenyl-based PSA prepared tape to delaminate.